Lithographic inks consist primarily of pigments, natural and/or synthetic resins with high melting points (100.degree. C. to 200.degree. C.), alkyd resins, and hydrocarbon solvents. Low concentrations of plasticizers, antioxidants, chelates, pH modifiers, antiskinning agents, and other additives also are included in lithographic ink formulations.
The natural and synthetic high-melting resins are typically either petroleum-derived or wood-derived. Used solely or in combination, these resins are dissolved in the high-boiling hydrocarbon solvents to give homogenous systems well known in the art as varnishes. Varnishes usually contain 20 to 70% resin solids. The alkyds, plasticizers, antioxidants, etc. are often included in the varnish, so that solids levels may exceed 70%.
It is known to those skilled in the art that in order to prepare phenolic resins, rosin may be reacted with phenols and formaldehyde to give phenol-rosin condensates which contain reactive double bonds and which may be reacted further in Diels-Alder and/or "ene"-addition fashion with alpha, beta-unsaturated carbonyl compounds. These Diels-Alder and ene adducts of phenol-rosin condensates are then esterified with polyols to give broad molecular weight distribution resins with low residual acid numbers. The traditional methods of producing such phenolic rosin resins are well-known. Generally, the rosin is first heated (to around 180.degree. C.) and melted. Thereafter the temperature is reduced (to around 115.degree. C.), phenol and formaldehyde are added, and the mixture is reacted in a phenol-condensate reaction over a period of time (normally around two hours). The resulting product is held under pressure for about two hours, then vented. The temperature is subsequently elevated to about 195.degree. C., maleic anhydride (or the like) is added, and the resulting mixture is maintained at temperature for around an hour and a half. Thereafter a polyol is added to the mixture, the temperature is increased to about 275.degree. C. and maintained for eight to ten hours in order to produce the phenolic rosin resin.
Phenolic resins must meet with several general requirements to be useful as lithographic ink resins. In order to make varnishes, for example, they must be capable of being dissolved in high-boiling hydrocarbon solvents to yield clear varnishes with manageable viscosities for easy workability. The varnishes must be stable in storage to viscosity, color, and clarity changes. On paper, the resin in the varnish or finished ink must dry to yield a durable, smooth, and uniform film with good resistance to abrasion and chemicals.
Moreover, it is appreciated that for phenolic resins to be useful as dispersing resins in lithographic ink pigment processing operations such as flushing, the resins must exhibit several specific properties in addition to the aforementioned requirements general to all lithographic ink resins. For example, when mixed with highly aqueous pigment presscake in high torque dough mixers commonly used for flushing operations, the resins present in the lithographic ink varnish must exhibit excellent pigment wetting properties. Such properties lead to rapid and thorough coverage of pigment particles present in the presscake and to the concurrent displacement of water originally bound to or entrained in the particle aggregates and agglomerates. Good wetting properties also lead to strong adhesion of resin to particle surfaces so that, as aggregates and agglomerates are broken down into primary particle units, resin will coat the particle surfaces thereby providing a steric barrier to particle-particle reaggregation and reagglomeration. Strong adhesion to and thorough coverage of surfaces of primary particle units by resin thus leads to increased color strength, gloss, and transparency, as well as reduced bronzing in the resulting pigment concentrate.
However, major problems exist with the traditional methods of producing phenolic rosin resins. For example, the condensation reactions associated with these methods often result in excessive foaming. Such foaming can lead to spillages and the release of excessive amounts of aldehyde vapors into the atmosphere. While it is a common practice in the industry to employ anti-foaming agents in an attempt to control this problem, the use of such agents adds to the cost of the process. Moreover, these agents often cause printing inks formulated with the resulting phenolic rosin resin compositions to have adhesion problems (which are known in the industry as "fish eyes").
Another problem associated with these methods is that the phenolic condensation reaction temperatures must be maintained above about 110.degree. C. Should the temperatures drop below this level, the rosin can crystallize--thereby forming a gel and rendering the mixture unusable. Yet a further problem associated with these traditional methods is the excessive cycle times caused by the necessity of adding the aldehyde at a controlled rate.
Therefore, an object of this invention is to solve these major problems by disclosing an improved method of producing phenolic rosin resin compositions that exhibit properties which makes them useful in formulating vehicles for lithographic printing inks and other coating applications.
A further object of this invention is to produce novel phenolic rosin resin compositions which can be employed in varnishes for use in formulating lithographic printing inks.